Energy absorbing devices have taken a wide variety of forms in automobiles. In general, such devices have been connected to the front bumpers to absorb impacts to minimize damage to the main body of the automobile as well as providing protection for the passengers. An automobile involving such energy absorbing devices is described in a copending patent application entitled "Front and Rear Energy Absorbing Structures for a Four Passenger Vehicle", Ser. No. 344,731, filed Feb. 1, 1982, and assigned to the same assignee as the present invention.
In a frontal collision of an automobile, it is desirable to absorb the kinetic energy of the moving vehicle (Kinetic Energy=(w/2 G)V.sup.2) over a long crush distance (S) of the frontal structure (Energy absorbed=FS), where w is the weight of the vehicle, g is the gravity and F is the force. This long crush distance is desired to reduce the crush force (F) and the acceleration level, (F=(w/g)a), where a is the acceleration involved.
As an explanatory example, the kinetic energy of a 2000 pound vehicle traveling at a speed of 40 mph (58.7 feet per sec.) is: ##EQU1## The maximum desirable acceleration is 36 g's as established by many tests on humans in aircraft and missle systems. This then determines the force level. ##EQU2##
At this maximum crush force, or crush resistance of the frontal structure, the crush distance will be ##EQU3##
The distance from the firewall to the bumper beam varies from vehicle to vehicle but in subcompact sizes of 2000 pound weight this distance is 36 inches or less. Within this space the engine and other equipment must be mounted. The engine in particular acts as a large uncrushable object and as such reduces the distance for crushing in a collision. An an example, if the engine width is 20 inches then the available crush distance remaining is 36-20 or 16 inches. The available crush distance is 1.9 inches less than the desired crush distance.
Within the front of the vehicle the two front rails (sills), extending from the firewall to the front bumper, provide the major portion of the crush resistance. The front bumper is attached to these two rails in the front.
The front bumper system must withstand a 5 mph impact into a barrier without suffering any permanent damage and the vehicle behind the bumper must also be undamaged. To provide such a bumper system, energy absorbing systems capable of stroking two inches at forces of 10,000 pounds are required. The total depth of such devices are usually 4 inches minimum, consisting of a flexible foam or a thermoplastic (polycarbonate) shell mounted to an aluminum alloy or steel box beam. This box beam is in the order of 2 inches deep to minimize deflections and optimize the weight. The box beam alone will collapse with little or no energy absorption in a frontal collision of 30 mph or greater.